This invention relates to a process for making additional interconnect layers on commercially available printed circuit boards using toughened benzocyclobutene based polymers and metal foils coated with such polymers. This invention also relates to such toughened benzocyclobutene-based polymers.
Various printed wiring boards or printed circuit boards (referred to herein as xe2x80x9cPWBsxe2x80x9d) are known and frequently have two to eight layers of laminates. Occasionally, it is desirable to add additional layers to these board structures (see, e.g., H. T. Holden, In Printed Circuits Handbook, 4th ed., C. F. Coombs, Jr., Ed.; McGraw-Hall: New York, 1996, Chapter 4). For high frequency applications, the insulating materials typically used in the PWBs (e.g., epoxy materials) may not provide the desired low dielectric loss. Therefore, it would be desirable to have a process and materials which could provide the desired low dielectric loss.
Benzocyclobutene-based polymers (hereinafter referred to as xe2x80x9cBCB polymersxe2x80x9d) are thermosetting polymers. Certain of these polymers have low dielectric constants and thus, are desirable as insulating coatings in various electronics applications (see, e.g., xe2x80x9cBenzocyclobutene(BCB) Dielectrics for the Fabrication of High Density, Thin Film Multichip Module,xe2x80x9d Journal of Electronics Materials, Vol. 19, No.12, 1990 and G. Czornyl, M. Asano, R. L. Beliveau, P. Garrou, H. Hiramoto, A. Ikeda, J. A. Kreuz and O. Rohde, In Microelectronic Packaging Handbook, Volume 2; R. R,.Tummala, E. J. Rymaszewski and A. G. Klopfenstein, Eds.; Chapman and Hall: New York, 1997; Chapter 11). However, for certain applications, BCB polymers may not have sufficient toughness. Therefore, it would be desirable to have a BCB polymer with increased toughness without significant loss or degradation of other desirable properties.
JP 8-60103 discloses an adhesive sheet comprises a compound having a benzocyclobutene group and a binder polymer. The preferred binder polymers have carbon-carbon unsaturated bonds. The reference indicates that from about 1-95 percent of the composition may be the binder polymer.
According to a first embodiment, the present invention is a process comprising
providing a printed wiring core board, which comprises a glass fiber reinforced board with conductive metal circuitry,
laminating to the printed wiring core board a sheet comprising a foil of a conductive metal and, on the foil, a dielectric layer of a curable composition comprising (a) at least one precursor compound selected from arylcyclobutene monomers, arylcyclobutene oligomers, and combinations thereof; and (b) a polymer or oligomer having a backbone comprising ethylenic unsaturation (i.e., carbon-to-carbon double bond(s)), wherein, during the lamination step, the dielectric layer is in contact with the printed wiring board, and
processing the laminated article to form additional electrical connections.
According to a second embodiment, this invention is a preferred composition comprising
a) at least one precursor compound selected from arylcyclobutene monomers, arylcyclobutene oligomers, and combinations thereof; and
b) a polymer or oligomer having a backbone comprising ethylenic unsaturation (i.e., carbon-to-carbon double bond(s)) and terminal acrylate or methacrylate groups. The invention is also the partially polymerized (b-staged) reaction product of this composition and the cured reaction product of the b-staged material.
According to yet a third embodiment, the invention is a sheet comprising a metal foil and a film comprising the composition of the second embodiment or the partially cured product of that composition.
The core board to which the BCB coated metal foil is laminated is characterized by having various insulating layers laminated together and separating various metal interconnect lines. Processes for making such PWBs are well-known (see, e.g., Printed Circuits Handbook, 4th Ed., C. F. Coombs, Jr., McGrall-Hill: New York, 1996). Typically, the insulating layers are glass reinforced epoxy. The metal foil is a very thin layer of a conducting metal, such as Cu or Cu alloy, and is preferably copper. The thickness of the metal foil is preferably about 3 to 50 microns. The BCB dielectric layer is preferably coated from a solvent onto foil. Suitable solvents include mesitylene xylenes, toluene, methyl ethyl ketone, cyclic ketones and a mixture of these solvents. After coating, the composition is preferably baked at temperatures from 100xc2x0 C. to 200xc2x0 C., more preferably 120xc2x0 C. to 180xc2x0 C. in air for 5 to 30 minutes, most preferably at 140xc2x0 C. to 160xc2x0 C. for 10 to 20 minutes.
Lamination to the core board may occur according to standard processes, preferably by vacuum hot press at temperatures of 200xc2x0 C.-250xc2x0 C. and pressures of 10-40 Kg/cm2.
Additional patterning steps may then be performed to the laminate according to known procedures. For example, the metal foil could be stripped or pattern etched followed by via formation with a laser drilling (e.g. with a CO2 laser), or the entire laminate could be mechanically drilled through. Electroless plating of additional conductive metal, such as copper, followed by electroplating (e.g. with copper) could be used to form the desired electrical connections.
As stated above, the precursor is either an arylcyclobutene monomer, a b-staged oligomer of one or more arycyclobutene monomers, or some combination of b-staged arylcyclobutene oligomers and/or monomers.
Preferably, the monomers are of the formula 
wherein
B1 is an n-valent organic linking group, preferably comprising ethylenic unsaturation, or B1 is absent. Suitable single valent B1 groups preferably have the formula xe2x80x94CR8xe2x95x90CR9Z, wherein R8 and R9 are independently selected from hydrogen, alkyl groups of 1 to 6, most preferably 1 to 3 carbon atoms, and aryl groups, and Z is selected from hydrogen, alkyl groups of 1 to 6 carbon atoms, aryl groups, xe2x80x94CO2R7 wherein R7 is an alkyl group, preferably up to 6 carbon atoms, an aryl group, an aralkyl group, or an alkaryl group. Most preferably Z is xe2x80x94CO2R7 wherein R7 is an alkyl group, preferably up to 6 carbon atoms, an aryl group, an aralkyl group, or an alkaryl group. Suitable divalent B1 groups include xe2x80x94(CR8xe2x95x90CR9)oxe2x80x94(Zxe2x80x2)oxe2x88x921, wherein R8 and R9 are as defined previously, o is 1 or 2, and Zxe2x80x2 is an alkyl group of 1 to 6 carbon atoms, an aromatic group, or a siloxane group. Most preferably o is 2 and Zxe2x80x2 is a siloxane group.
Ar1 is a polyvalent aromatic or heteroaromatic group and the carbon atoms of the cyclobutane ring are bonded to adjacent carbon atoms on the same aromatic ring of Ar1, preferably Ar1 is a single aromatic ring;
m is an integer of 1 or more, preferably 1;
n is an integer of 1 or more, preferably 2-4, more preferably 2; and
R1 is a monovalent group, preferably hydrogen, lower alkyl of up to 6 carbon atoms.
The synthesis and properties of these cyclobutarenes, as well as terms used to describe them, may be found, for example, in U.S. Pat. Nos. 4,540,763; 4,724,260; 4,783,514; 4,812,588; 4,826,997; 4,999,499; 5,136,069; 5,185,391; 5,243,068, all of which are incorporated herein by reference.
According to one preferred embodiment, the monomer (a) has the formula 
wherein
each R3 is independently an alkyl group of 1-6 carbon atoms, trimethylsilyl, methoxy or chloro; preferably R3 is hydrogen;
each R4 is independently a divalent, ethylenically unsaturated organic group, preferably an alkenyl of 1 to 6carbons, most preferably xe2x80x94CH2xe2x95x90CH2xe2x80x94;
each R5 is independently hydrogen, an alkyl group of 1 to 6 carbon atoms, cycloalkyl, aralkyl or phenyl; preferably R5 is methyl;
each R6 is independently hydrogen, alkyl of 1 to 6 carbon atoms, chloro or cyano, preferably hydrogen;
n is an integer of 1 or more;
and each q is an integer of 0 to 3.
The preferred organosiloxane bridged bisbenzocyclobutene monomers can be prepared by methods disclosed, for example, in U.S. Pat. Nos. 4,812,588; 5,136,069; 5,138,081 and WO 94/25903. The preferred compound where n is 1, q is 0, R4 is xe2x80x94Cxe2x95x90Cxe2x80x94, R5 is methyl, and R6 is hydrogen is referred to herein as DVS-bisBCB.
If an oligomeric precursor is desired, the BCB monomers may be b-staged according to any known process. The monomers may be partially polymerized or b-staged neat (i.e., without solvent) by heating (see, e.g., U.S. application Ser. No. 08/290,197 and U.S. Pat. No. 4,642,329, incorporated herein by reference). Alternatively, the monomers may be partially polymerized or b-staged in a solvent (see, e.g., U.S. application Ser. No. 08/290,197, incorporated herein by reference). When oligomeric precursors are used, the weight average molecular weight (Mw) is preferably less than 200,000, more preferably less than 150,000 and preferably greater than 10,000, more preferably greater than 20,000.
The second component having the ethylenic unsaturation in the carbon backbone should be selected so that it can withstand the processing conditions (solvents, heating, etc.) used in microelectronics fabrication and not cause a significant deterioration in the electrical insulating properties of the reaction product relative to an unmodified BCB polymer. Suitable materials include polymers based on butadiene, isoprene, ethylene-butene, and ethylene-propylene. Comonomeric units, such as styrene and methylstyrene, may also be used. Preferably, the ethylenically unsaturated polymer is selected from polybutadiene, polyisoprene, styrene-butadiene block copolymers, and styrene-isoprene block copolymers. Applicants have found that the terminal groups on the ethylenically unsaturated polymer can have a profound effect on the performance of the composition. Acrylate or methacrylate terminated polymers are the preferred materials, as recited in the second embodiment of this invention, with acrylate terminated polybutadienes being the more preferred. The most preferred polymer has the formula 
wherein l, m and n represent the mole fraction of the respective group in the polymer and (l+n) is from about 0.4 to about 0.95 and m is from about 0.05 to about 0.6, R and Rxe2x80x2 are independently in each occurrence alkyl groups of 1 to about 10 carbon atoms, and preferably are methyl groups.
The molecular weight (Mw) of the second component is preferably less than 150,000, more preferably less than 100,000, most preferably less than 80,000, and preferably greater than 3,000, more preferably greater than 5,000.
The amount of the second component used in the composition should be such as to avoid excessive phase separation between the first and second components. When phase separation occurs significantly, it depends upon various factors, such as the molecular weights of the components, the characteristics and relative amounts of any comonomers, the solvents used, and the temperature of processing. Preferably, the amount of the second component is less than 50 parts per hundred parts of the arylcyclobutene material (phr). For the composition of the second embodiment (the preferred composition having acrylate terminal groups), the second component is preferably present in amounts of at least 20 phr to give the maximum combination of toughness, flexibility and dimensional stability. For other non-preferred embodiments, the amount of the second component is preferably less than 15 phr to avoid phase separation and solubility problems.
The composition may be cured by any known method, such as, for example, by heating to temperatures from 200xc2x0 C. to 300xc2x0 C. Frequently, the composition is coated on a substrate (e.g., by spin coating, or drawing with a bar) to yield a film, and dried and cured by heating.
If desired, a photosensitive compound may be added to render the composition photosensitive. A coated film of the composition can then be patterned by exposure to activating wavelengths of radiation. Photosensitizers that increase the photoactive compound""s photosensitivity may also be added. Any photoactive compounds and photosensitizers that are known in the art may be used. Examples of photoactive compounds include bisazides, a combination of bismaleimides, acrylates, acetylenes, and radical initators such as 2,2-dimethoxy-2-phenylacetophenone. The amount of photoactive compound is preferably 0.1 percent to 20 percent by weight, more preferably about 0.5 percent to 8 percent, based on total weight of components a and b and the photoactive compound. See U.S. application Ser. No. 08/290,197, incorporated herein by reference, for additional discussion regarding photocurable BCB compositions and methods of developing such compositions. See also U.S. Ser. No. 09/177,819, incorporated herein by reference.
Flame retardant compounds may be added to render the flame retardancy. Examples of flame retardant compounds include phosphate compounds, such as triphenylphosphate and trishaloethyl phosphate, halogenated compounds, such as polymerized tetrabromo-bis-phenol A and inorganic compounds such as magnesium and calcium carbonate. Multiple flame retardant compounds can be used to enhance the flame retardant effect. The amount of flame retardant compound is preferably 5 percent to 20 percent by weight, more preferably about 5 percent to 10 percent, based on total weight of components a and b and the photoactive compound.
Other components, such as antioxidants (e.g., quinolines, hindered amines, monomethyl ether hydroquinone, and 2,6-di-tert-butyl-4-methylphenol), adhesion promoters, additional cross-linkers (e.g., 2,6-bis(4-azidobenzylidene)-4-ethylcyclohexanone) (BAC-E) that are known in the art may also be included in the composition. Using additional cross-linkers that are reactive to BCB under dry, heated conditions are advantageous for the metal-coated foils because they enable one to control resin flow during the lamination process.
Suitable solvents for the composition include mesitylene, xylenes, toluene, methyl ethyl ketone, cyclic ketones and mixture of these.